Diabetes is characterized by peripheral insulin resistance, increased glucose production and a decrease in the levels of insulin secretion. In general the levels of glucose in the serum are elevated. Moreover, serum glucose levels are raised for a longer period of time after ingestion of meals, and return to normal at a reduced rate. The consequences of increased glucose levels are well known, although the biochemical and molecular mechanisms underlying these phenomenon have not yet been clearly defined. Free fatty acids, triglycerides and other factors can also directly lead to increased levels of glucose.
The hexosamine pathway has been linked as one of the biochemical pathways that can contribute to insulin resistance, increased glucose production, and decreased insulin secretion. The hexosamine pathway is involved the synthesis of UDP-GlcNAc. Glucose is sequentially converted to fructose-6-phosphate, glucosamine-6-phosphate, and eventually converted to UDP-GlcNAc. Once UDP-GlcNAc is synthesized, it is incorporated into a variety of glyco-containing macromolecules, many of which are key cellular components. In addition, UDP-GlcNAc is a substrate for the enzyme OGT, O-linked GlcNAc transferase, that catalyzes the transfer of GlcNAc residues to various proteins in the cell, including cytoplasmic proteins, nuclear proteins, membrane proteins, and transcription factors. In so doing, the activity of these proteins can be significantly modulated. The rate limiting enzyme in this pathway is glutamine fructose-6-phosphate amidotransferase (GFAT), which catalyzes the amido transfer and isomerization of fructose-6-phosphate to glucosamine-6-phosphate. GFAT has been implicated in the development of diabetic symptoms, as GFAT transgenic mice are insulin resistant. The biochemical pathways that lead to insulin resistance include activation of PKC, alteration of membrane components, altered transcriptional activity, as well as other biochemical mechanisms that remain to be elucidated.
GFAT levels are elevated in type 2 diabetes mellitus (T2DM) and in rodent T2DM models. GFAT transgenic mice (muscle, liver, adipose and pancreas specific) are both insulin resistant and hyperinsulinemic. Glucosamine and products of the hexosamine pathway cause insulin resistance, increased hepatic glucose output and decreased insulin secretion. GFAT may play a role in T2DM kidney complications. GFAT is the rate limiting enzyme in the hexosamine pathway, and decreasing GFAT enzymatic activity should result in glucose lowering and be beneficial in treating diabetes.
Known classes of GFAT inhibitors are substrate-like or non-substrate-like and are believed to inhibit by either reversible or irreversible (covalent) mechanisms. The two subtrates of GFAT are the saccharide, fructose-6-phosphate, and the amino acid, glutamine. Fructose-6-phosphate-like inhibitors include: N-iodoacetylglucosamine-6-phosphate (S. L. Bearne, J. Biol. Chem., 271, 3052–3057 (1996)), and 2-amino-2-deoxyglucitol-6-phosphate (M.-A. Badet-Denisot, C. Leriche, F. Massiere, and B. Badet, Bioorg. Med. Chem. Letters, 5, 815–820 (1995)). Glutamine-like or glutamine-based inhibitors include: glutamate-γ-semialdehyde (S. L. Bearne and R. Wolfenden, Biochem., 34, 11515–11520 (1995)), L-γ-glutamyl-2-[((p-difluoromethyl)phenyl)thio]-glycine (F. Massiere, M.-A. Badet-Denisot, L. Rene, and B. Badet, J. Amer. Chem. Soc., 119, 5748–5749 (1997)), anticapsin (H. Chmara, J. Gen. Microbiol., 131, 265–271 (1985)), 6-diazo-5-oxo-norleucine (DON), azaserine, and N3-haloacetyl-L-2,3-diaminopropanoic acid (where halo=I, Br, and Cl) (S. Milewski, H. Chmara, R. Andruszkiewicz, and E. Borowski, Biochim. Biophys. Acta, 1115, 225–229 (1992)).
Papaveraldine (CA Index Name: Methanone (6,7-dimethoxy-1-isoquinolinyl) (3,4-dimethoxyphenyl)-(9C1)) exhibits properties which implicate potential usefulness in the treatment of heart disease. (Anselmi, Elsa, et al., “Selective inhibition of calcium entry induced by benzylisoquinolines in rat smooth muscle”, J. Pharm. Pharmacol. (1992) 44(4), 337–43; Markwardt, Fritz, et al., “Influence of 6,7-dimethoxyisoquinoline derivatives on the function of thrombocytes”, Acta Biologica et Medica Germanica (1969) 23(2), 295–306).